The present invention relates to a method and apparatus for separating air to produce an oxygen product in which the air is separated in a double distillation column system having a higher pressure column and a lower pressure column reboiled at multiple levels thereof. More particularly, the present invention relates to such a method and apparatus in which part of the plant refrigeration requirements are produced by partially vaporizing crude liquid oxygen withdrawn from the bottom of the higher pressure column and expanding the resultant vapor.
There are many plant designs that can be employed for separating air by cryogenic rectification to produce an oxygen product. In one type of plant design, the air is initially separated in a higher pressure column to produce an oxygen enriched liquid column bottoms and a nitrogen rich tower overhead. The oxygen rich liquid column bottoms is further refined in a lower pressure column to produce a liquid oxygen column bottoms and a nitrogen vapor tower overhead. The liquid oxygen column bottoms is formed into a product stream that may be taken as a liquid and then either vaporized or pumped to a vaporization pressure against liquefaction of a boosted pressure air stream.
The lower pressure column is reboiled at several levels. The bottom level reboiling is effectuated by condensing at least part of the air to be separated. After such partial condensation, the air is introduced into the higher pressure column for separation. Nitrogen rich tower overhead of the higher pressure column is condensed in a head condenser and then used to reflux both the higher and lower pressure columns. Such condensation is effectuated by valve expanding a crude liquid oxygen steam, composed of the oxygen enriched liquid column bottoms of the higher pressure column, to form a coolant for the head condenser. The coolant is then used to fully condense nitrogen rich vapor taken from the higher pressure column.
It is known that the energy outlay involved in running such a plant is dependent upon the degree to which the air condenses within the lower reboiler located in the lower pressure column. The lower the degree of condensation, the lower the degree to which the incoming air must be compressed by the main air compressor. It has been found by the inventors herein, further energy savings can be realized that are obtainable in the prior art.